In recent years, there has been increasing interest in signature verification type security systems for providing access to individuals to either secured areas or more commonly to data banks and associated processors, on the basis of the individual's signature as executed at a pre-designated place with a specially provided pen.
One such system which has been found to provide advantageous results is disclosed in Herbst et al. U.S. Pat. No. 3,983,535, and is based on the discovery that when an individual signs his name, certain acceleration components are imparted to the pen, for example in the x and y directions, and that such acceleration components are repeated with each signature by the same individual, and may be the basis for identifying that individual's signature. In Herbst et al. U.S. Pat. No. 4,128,829 both pen acceleration and pen stylus pressure on the writing surface are used to identify signatures. In the particular system disclosed in that patent the sampled accelerations and pressure values are first segmented and are then correlated with corresponding segments of a reference signature. Successive comparisons are performed utilizing successive shifting of the phases between the segments being correlated to find regions of highest possible correlation. A running account of maximum cross correlation values is kept and the maximum values for each segment pair comparison are combined, with the resultant correlation value being utilized as a verification indicator.
It is evident that signature verification systems such as described above must include some means of determining pen acceleration. Since the acceleration imparted to the pen is proportional to the muscle forces which are exerted by the signer, one type of apparatus which has been utilized is the combination of a strain gauge transducer mounted in the pen and a resistance bridge. However, strain gauge devices have the disadvantage of generally being quite expensive, and also are difficult to mount and may be easily damaged by impact or excessive pressure.
Another type of transducer which has been utilized is the piezoelectric type, e.g., see U.S. Pat. No. 4,078,226. Piezoelectric transducers generate a signal upon being compressed and such signal may be used as a measure of the muscle forces which are exerted on the pen. However, problems involving variations in cable capacitance, and leakage, have made piezoelectric transducers difficult to use.
Herbst et al. U.S. Pat. No. 3,983,535 mentioned above suggests the use of a commercially available tablet device which produces analog signals proportional to x-y displacement. However, since it is necessary to provide an apparatus for generating the second derivative of the displacement output signal to obtain acceleration, such devices may be unduly complex.